What is the effect of vagal tone on heart rate?

• A. Decreases HR
• B. Increases HR
• C. No effect on HR
• D. Causes tachyarrhythmia

Answer: (A)

Vagal tone slows the heart rate by activating the parasympathetic system. When the vagus nerve releases acetylcholine, it acts on M2 muscarinic receptors in the SA and AV nodes. This opens potassium channels and increases K+ efflux, hyperpolarizing pacemaker cells and slowing the slope of phase 4 depolarization. The SA node fires more slowly, lowering the resting heart rate. It also lengthens AV nodal conduction and raises its refractory period, which helps regulate rhythm and can terminate certain fast rhythms via AV nodal blockade. So the effect is a decrease in heart rate with higher vagal tone. The other statements don’t fit: vagal tone does slow HR, it’s not neutral, and while it can help terminate some tachyarrhythmias, it does not cause tachycardia.